1. Field of the Invention
The disclosures herein relate to a surface emitting laser device and an atomic oscillator.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) is a semiconductor laser, which emits light in a direction perpendicular to a substrate surface. The VCSEL has a feature, compared with an end-face emitting type semiconductor laser, of low cost, low power consumption, small size, high performance, and being easy to be integrated two-dimensionally.
The vertical cavity surface emitting laser has a resonator structure that has a resonator region including an active layer. The resonator structure also has an upper reflecting mirror and a lower reflecting mirror provided above and below the resonator region respectively. The resonator region has a predetermined optical thickness so that light with wavelength of λ resonates in the resonator region in order to obtain light with an oscillating wavelength of λ. The upper and lower reflecting mirrors are formed by laminating materials having different refraction indices, i.e. a low refraction index material and a high refraction index material, alternately, and are formed so that the optical thickness of the low and high refraction index materials is λ/4, in order to obtain high reflectance where the wavelength is λ.
These upper and lower reflecting mirrors are formed with a distributed Bragg reflector (DBR), and, for example, in the case where GaAs substrate is used as the substrate, AlGaAs-based material, which is easily lattice-matched with the GaAs substrate, is often used. Generally, in a normal surface emitting laser, the lower reflecting mirror is often formed of an n-type material and the upper reflecting mirror is often formed of a p-type material.
The p-type semiconductor has high optical absorption according to inter-valence absorption and absorption between valence band acceptor levels. Because of this, a surface emitting laser using a p-type semiconductor layer for the light wave guide region may suffer an increase in threshold current, a decrease in external quantum efficiency, and degradation in temperature characteristics.
Japanese Published Patent Application No. H11-243257 discloses a surface emitting laser having a structure using DBRs including dielectric material instead of the semiconductor material. In this case, since the dielectric material is an insulating material, providing a current path bypassing the DBR formed of the dielectric material is necessary. Such a structure is generally called an intra-cavity type. Furthermore, in the DBR formed of the dielectric materials, difference in refraction index between the high refraction index material and the low refraction index material forming the DBR is larger than that formed of the semiconductor materials. Accordingly, the DBR formed of the dielectric materials can obtain a high reflection rate even with a small number of pairs, compared with the DBR formed of the semiconductor materials. Therefore, the surface emitting laser has an advantage that the size in the laminating direction becomes small.
Furthermore, a structure in which a multi-wavelength surface emitting laser with different wavelengths is formed in one chip is also disclosed, and is expected to be widely used for multi-wavelength communication and the like. As a method of obtaining such a multi-wavelength surface emitting laser device, Japanese Patent No. 2751814 discloses, for example, a structure, in which an oscillator region of the surface emitting laser is formed with a wavelength adjusting layer formed by alternately laminating two kinds of layer which are etched by different kinds of etching liquid. Specifically, the structure is obtained by removing one layer for each surface emitting laser by wet etching, thereby changing a layer thickness of the wavelength adjusting layer, and the layer thickness in the resonator region is changed for each surface emitting laser. The surface emitting laser produced by such a method is regarded as preferable from a viewpoint of controllability of wavelength of laser light emitted from the surface emitting laser and from a viewpoint of productivity.
In an apparatus where the surface emitting laser is used, light emitted from the surface emitting laser (emitted light) may be required to have a polarization of a predetermined direction. However, since the surface emitting laser is different in structure from the end-face emitting laser, it is difficult to set the polarization in the predetermined direction. Accordingly, a variety of methods of setting polarization in a predetermined direction have been studied. For example, Japanese Published Patent Application No. 2008-98338 discloses a surface emitting laser, in which on an active layer, a stress adding layer applying stress in a predetermined direction within a laminating surface to the active layer is formed. However, in such a surface emitting laser, since polarization direction is controlled by the stress in the layer, there is a problem that the polarization direction is not sufficiently stabilized.
The present invention has been made in view of the above circumstances, and it is a general object of at least one embodiment of the present invention to provide a surface emitting laser device which emits laser light with stable polarization in a predetermined direction.